Flow, Mixing and Heat Transfer in Furnaces. The Role of Meaningfulness, Similarity, and Familiarization

243 Pages·1978·14.02 MB·English

by  K.H. Khalil

HMT
THE SCIENCE & APPLICATIONS OF HEAT AND MASS TRANSFER
Reports, Reviews & Computer Programs
Editor-in-Chief:
D. BRIAN SPALDCVG,
Mechanical Engineering Department,
Imperial College of Science and Technology,
Exhibition Road,
London SW7 2AZ, England
ALSO IN THIS SERIES
Volume 1 —SPALDING: GENMIX — A General Computer
Program for Two-dimensional Parabolic
Phenomena
Flow, Mixing and Heat
Transfer in Furnaces
Edited by
K. H. KHALIL, F.LMechE.
Professor and Head of Mechanical Engineering Department,
Faculty of Engineering, Cairo University, Cairo
Assisted by
F. M. EL-MAHALLAWY and E. E. KHALIL
(ASSOCIATE PROFESSOR) (LECTURER)
Mechanical Engineering Department,
Faculty of Engineering, Cairo University
Editor-in-Chie/
D. B. SPALDING
Department of Mechanical Engineering,
imperial College of Science and Technology, London
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First edition 1978
British Library Cataloguing in Publication Data
Conference on Mechanical Power Engineering, 1st,
Cairo University, 1977
Flow, mixing and heat transfer in furnaces.
- (HMT, the science and applications of heat and
mass transfer ; vol. 2).
1. Furnaces - Congresses 2. Heat - Transmission -
Congresses
I. Title II. Khalil, K H III. El-Mahallawy,
FM IV.Khalil,EE V. Series
621.4Ό25 TJ320 77-30754
ISBN 0-08-022695-7
In order to make this volume available as economically and as rapidly as
possible the authors' typescripts have been reproduced in their original forms.
This method unfortunately has its typographical limitations but it is hoped that
they in no way distract the reader.
Printed in Great Britain by
Cox & Wyman Ltd, London, Fakenham and Reading
PREFACE
This volume mainly comprises a selection from papers presented at the
1st Conference on Mechanical Power Engineering, held at Cairo University in
February 1977. The papers describe experimental and theoretical research
work carried out in the field of flow,mixing and heat transfer in furnaces.
Because of the close association between the departments of Mechanical Eng-
ineering in Cairo University and the Imperial College of Science and Tech-
nology, University of London, a few contributions from the research group
in the same field at Imperial College are included in this volume.
Furnace and combustion-chamber designers seek performance of high
combustion efficiency and low pollution formation. The major aim of many
investigations is to provide information which is quantitatively useful to
designers, and which will replace the "rule-of-thumb" approach commonly
used. This information should allow the calculation of local furnace-flow
properties,wall heat transfer and pollution formation rate on a firm quant-
itative basis. However, fundamental understanding of mixing and reaction
processes and detailed experimental data are both deficient, because of the
complexity of the processes in furnaces and combustors.
Experimental and theoretical investigations are complementary, and
necessary to enhance the understanding of the flow,mixing and heat trans-
fer in furnaces and combustion chambers. Reliable measurements of time-
averaged values of aerodynamic, chemical and thermodynamic quantities which
characterise turbulent flames, performed in systems identical with or
similar to industrial furnaces, are a great asset to the designer. The determ-
ination of the local flow properties, turbulence intensity, gas temperature,
species concentrations and wall heat fluxes forms the principal objective
of the experimental investigations described in this volume. To perform
these measurements, standard measuring techniques were used and modified to
reduce possible sources of error. These techniques include pitot probes
for mean velocity measurements, thermocouples and pyrometers for mean gas
temperature measurements, sampling probes for species concentration measure-
ments and ellipsoid radiometer for radiative heat fluxes.
However, experimental research is time-consuming and very expensive,
as extensive measurements are needed to cover all the factors affecting
the investigated flow. An alternative route is desired to lead the designer
to the information he requires : prediction procedures can provide such an
alternative. Once a computational procedure is established, then the detailed
experiments can be dispensed with.
The prediction procedures are of two kinds. The first type involves
scale modelling while the second involves computer modelling. Scale model-
ling is not very practical as many of the investigated flow properties do
not scale to furnace major dimensions; also there are significant discrep-
ancies between measurements and predictions, particularly in the vicinity
of the burner exit. Therefore the attention of research workers in the
field of the development of computational procedures is directed towards
computer modelling.
The computer modelling prediction procedures are only valuable when
their reliability has been established. Their reliability can be confirmed
only by testing their performance in practical situations. The validation
of the prediction procedure is a primary concern among research workers
in the field of combustion. The majority of papers contained in this volume
vii
viii Preface
are intended to contribute to the validation exercise, or at least can be
used in the validation process. Some of the papers describe measuring
techniques and present the results of the measurements that can be used
in validation tests. Some of the papers present only predictions of flow
pattern and heat transfer in furnaces. The more complete papers present
comparisons between predictions and experiments, which give insight on
the performance of the prediction procedures.
The computation procedures solve the conservation equations governing
the transport of mass, momentum and energy, expressed in a finite-differ-
ence form, in a domain that conforms to the geometry of the flow under
investigation. These procedures all involve various models of turbulence,
combustion and radiation. Some of these models represent the investigated
flows more adequately than others, therefore it is important to distin-
guish between them. Various models are represented among the papers; and
although the information included in the volume does not permit the discus-
sion of the merits and disadvantages of every model, they make significant
advances possible.
The material presented in this volume describes different aspects
of flow, mixing and heat transfer in confined flows under reacting and
non-reacting flow conditions. The papers investigating non-reacting flows
deal with problems of flow and mixing of two coaxial air streams under
various degrees of swirl, and investigate the effects of the different
geometrical and operating conditions on mixing and flow patterns in
furnaces and combustion chambers. Special attention is given to the
measurements of mixture fractions and the mechanism of jet spread, mixing
and flow separation.
The largest part of this volume is devoted to the problems of mix-
ing, combustion and heat transfer in reacting flows. It covers measuring
techniques together with prediction procedures in gaseous and liquid
spray flames in axisymmetric furnaces and combustion chambers. The exp-
erimental and numerical methods for the determination of local flow
properties and heat transfer characteristics are presented, and in some
papers comparisons between measured and calculated properties are
performed. Emphasis is focused on the determination of the heat rele-
ase rate and the radiative and convective heat transfer to the furnace
walls under various geometrical and operational conditions. Radiative
properties of the reactants and products of combustion are presented
in gaseous and liquid fuel spray combustion.
Most of the papers that use numerical procedures to calculate
local flow properties are applied to axisymmetric (two-dimensional)
furnace flows. A short contribution from Imperial College presents
a pioneer attempt to calculate three-dimensional recirculating non-
reacting flows.
K.H.Khalil, Editor
F.M.El-Mahallawy, Associate Editor
E.E.Khalil, Associate Editor
THEORETICAL AND EXPERIMENTAL STUDY OF
MIXING OF TWO CO-AXIAL CONFINED JETS IN
A COLD MODEL FURNACE
A. K. Khalil, F. M. El-Mahallawy and N. M. Raf at
Faculty of Engineering, Cairo University, Egypt
ABSTRACT
An experimental and theoretical investigation of the aerodynamic mixing pattern
of two co-axial confined jets, with special reference to fuel-air mixing in
furnaces, was carried out. Data were obtained for the parameters : mass ratio
of the two jets, the annular thickness separating the two jets, the Rsynolds
number and the degree of swirl or the swirl number of the annular jet. A com-
prehensive study was made on the effect of these parameters on mixing and dis-
tribution of constant concentration contours. Some predictions were made with
a simple turbulence model and compared with experimental data.
INTRODUCTION
The mixing of two co-axial confined jets is of special importance to fuel-air
mixing in furnaces· The phenomenon of flame in furnaces is a result of complex
interaction of physical and chemical processes. The knowledge of how to control
and predict the major properties of the flams has become essential to furnace
design engineers for obtaining optimum designs and to achieve maximum economy.
As an intermediate step to understand the mixing pattern in furnaces, which
depends on the rate of molecular and turbulent transfer, it is necessary to
study the aerodynamic mixing pattern without chemical reaction. This study
beside having the advantage of simplicity and accuracy in flow measurements,
it provides the necessary experimental data in order to develop the prediction
methods and finally to understand the actual mixing process accompanied by
chemical reaction.
In the present work, a cylindrical model furnace involving no chemical reaction
is designed and constructed to provide experimental data on the mixing between
two jets to simulate the actual mixing process in the hot model· This model
furnace is provided with a double concentric jet burner.
The main parameters investigated are : the mass ratio of the two jets, the
annular thickness separating the two jets, the Iteynolds number and the degree
of swirl of the annular jet.
EXPERIMENTAL WORK
The Test Rig
The test rig consists of a cylindrical model furnace provided with a double
concentric jet burner. The dimensions of the furnace cylinder are 0.205m dia-
meter, 2.15 m long and 6 mm thickness; Fig. 1 shows a sketch of the test rig.
1
2 Flow, Mixing and Heat Transfer in Furnaces
A double concentric jet burner is mounted on the model inlet· The primary air
is supplied through the central jet and its temperature controlled by an electric
heater. The secondary air is supplied through an air distributor to the annular
space surrounding the primary jet.
l. Furnace cylinder
2. Double concentric burner
3. Swirler
4. Secondary air distributor
5. Primary air stream
6. Electric heater segments
7. Orifice plates
8. Secondary air piping
9. Control valve
10. Overflow valve
11. Air blower
12. Electric motor
Fig. 1. Diagrammatic layout of the test rig.
Concentration Measurements
There is a number of tracer techniques used in concentration measurements :
1 Flow visualization for isothermal modelling with air or water.
#
2. For quantitative model investigation of velocity, static pressure and con-
centration, tracers are used. The tracer can be temperature (e.g., the
central jet can be preheated and the local concentration can be determined
by temperature measurement). Helium and carbon dioxide are also used as
tracers by being injected with one of the two jets and the local concentra-
tion at any point can be determined by analysing the sample at this point.
The tracer technique used here is the temperature tracer technique, The local
concentration at any point is calculated as follows :
t - t l-HI^/top
p s
Mixing of Two Co-axial Confined Jets 3
where F is the mixture fraction or the concentration of the primary jet,
t and tp are the temperature of the secondary and primary streams respectively
8
1% and mp are the local masses·
THE PREDICTION PROCEDURE
The mixing pattern in furnaces is predicted by solving the conservation elliptic
partial differential equations·
The fundamental laws of conservation of mass, energy and momentum provide the
partial elliptic differential equations, as well as some auxiliary relations
for thermodynamic transport properties plus boundary conditions make the mathe-
matical problem complete.
The predictions were obtained by using the computational procedure of Gosman
et al.(l), which solves the elliptic partial differential equations of the form
>._(^^)_1- ΐΤ)-·>-(Γ.Γΐ?)-^-(Γ.Γ^) + S -0
(ίϊ
~b z ""& r "or "fc z *dz 0 "^ z *br 0 "^ r <jf
where r and z are the radial and axial co-ordinates, y/ is the stream function,
0 is a dependent variable, S^ represents the source or sink of 0, and J"tf ie the
effective exchange coefficient, equations were solved with 0 standing in turn
fory, vorticity divided by radius to/r and the mixture fraction F.
The effective viscosity of the turbulent flow was determined from the simple
formula proposed by ïtef. 1;
I
Ueff - K D2/3 IT* S2/3(mf V* ♦ m vj) J
&
where D and L are the furnace diameter and length, ? is the density, mf and nu
are the mass flow rates of fuel and air, Vf and V are the corresponding vel-
a
ocities, and K is a constant assigned the value 0·012.
It requires to be stressed here that more satisfactory turbulence models now
exist. A particularly popular model involves the solution of two extra partial
differential equations such as equations for the turbulence kinetic energy and
its dissipation rate (2 & 3)· Here, we want only to demonstrate what can be
achieved when a simple turbulence model is employed, if high accuracy is not of
paramount importance, such models allow useful trends to be predicted with a
corresponding saving in computational time.
JESULTS
Effect of the Mass Ratio of the Two Streams
One of the important parameters that affects the mixing pattern in furnaces and
determines the dimensions of the flame is the fuel-air ratio in the hot system
with combustion or the mass ratio of the primary and secondary air jets in the
cold system.
4 Flow, Mixing and Heat Transfer in Furnaces
The distribution of the mixture fraction across the furnace for two mass ratios
is shown in Figs· 2 & 3 · This distribution was found to be of an exponential
relation of the general form;
■*Φ
2
Κ e
χ
F„
where F is the mixture fraction at a radius r, F is the mixture fraction on
m
centre line at a distance z from the burner, K^ and Kg are constants·
O SECTION AT 80 m. m. FROM BURNER
9 .. 130 - M
B ·· 230 ·· ..
Δ M 330 ·· " "
X " 480 " •I M
• M 605 ·· ..
A " 830 ·· •1 ··
I "1005 ·· ..
0.1
β
60 80 100
RADIUS mm
Fig· 2. Distribution of mixture fraction across the furnace at
zero0 swirl (1% » 405 Kg/hr, X - 7.1)·
Figure 4 shows a comparison of the constant concentration contours between the
two ratios of 5.21 and 9.7· If we represent the stoichiometric mixing ratio in
the hot tests with a suitable degree of mixing in the cold tests, it becomes
clear that the flame length in the hot system will get shorter as the mass ratio
of the two streams is increased·
The mixing length (determined fcy the point of intersection of the constant con-
centration contour with the model centre line) for a given degree of mixing is
plotted against the mass ratio of the two streams as shown in Fig· 5. The
different curves for different values of centre line mixing ratio are extrapolat-
ed to the origin· It is clear that at small values of the ratio of primary to
secondary mass flow rates, the curves in this origin are nearly straight lines*
Mixing of Two Co-axial Confined Jets 5
032
âSECTKJW AT 60 m.m. FROM BURNER
• ·» *· 130 m.m
o ·· 230 m.m
X ·· " 330 m.m
0.24 a " 360 m.m
Δ ·· 460 m.m. ·· ·· 1
■ ·· 605 m.m. ··
·· 755 m.m. ··
| — A -J
i f -4
0.16
0.08 X^w ——. ■ ^-* A
^ A I
0.001—
20 40 60 80 100
RADIUS mm
Fig. 3. Distribution of mixture fraction across the furnace
for zero0 swirl (nig « 405 Kg/hr, X - 5.21).
Fig. 4. Effect of the ratio of the two streams on the
constant concentration contours at zero 0 swirl
(m - 405 Kg/hr).